Ceramic capacitor polarity trick

It was this one:

Hi Walter,

Well that's certainly at the far end of "less" of "more or less" like a short circuit, eh? ;-) Thanks for the clarification. I've seen people suggest the "trick" of using two non-polarized caps back to back for years now, but never any particularly discussion of just how well it does or doesn't work.

---Joel

In message , dated Sat, 5 Aug 2006, Joel Kolstad writes

It works. Packaged 'reversible' elcos are sometimes made that way. The wrongly-biased section never gets the chance to pass damaging leakage current, because that is stopped by the correctly-biased section.

But the behavior will be complex. Very low-frequency capacitance will effectively be about 2x the high-frequency value, with an obvious nonlinear/distortion zone somewhere in the middle. They're OK as long as the AC swing across them is low.

John

In message , dated Sat, 5 Aug 2006, John Larkin writes

Where does non-linearity come in?

Of course; that's what capacitors in series with the signal path are designed to have. If there is any appreciable voltage across such a cap, it's value is too small.

The back-biased cap leaks with a current that's nonlinearly dependent on voltage. Any cap whose C is a function of V (even a linear function) generates harmonic currents, and the DC capacitance here is twice the small-signal AC capacitance.

Right. So they aren't ideal in filters or crossovers.

John

In message , dated Sat, 5 Aug 2006, John Larkin writes

The current is controlled by the insulation resistance of the correctly-oriented capacitor in series.

That's for DC. Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

In message , dated Sun, 6 Aug

Yes; the alternating current doesn't damage the dielectric. What comes off on one half-cycle goes back on during the next. If that wasn't so, power supply filter capacitors in compact fluorescent lamps wouldn't work: they work with a large ripple voltage so as to reduce mains harmonic current emissions.

Certainly. But apply a DC voltage to a discharged capacitor of this sort. Eventually, most of the voltage will wind up across the conventionally-biased section, with much less across the back-biased one. So the effective capacitance (computed from integrated current and final voltage) is twice the small-signal AC capacitance, and the path to "twice" is complex... the charging curve won't be anything like that of an ideal capacitor.

John

In message , dated Sun, 6 Aug 2006, John Larkin writes

I've measured some (discharge time through a known resistor), and it doesn't happen with my samples. I'd forgotten that I'd done that, but I've just checked my notes.

That depends on if you're using conventional rotation or electron rotation. ;-)

Cheers! Rich

In message , dated Sun, 6 Aug 2006, Apostrophe Police writes

OK, I did a typo. I do check such things, but since I'm outputting about

only for the first half-cycle after that you have the normal capacitance.

to put it another way. if you take the DC voltage away only half the charge will come out.

Bye. Jasen

that effect only occurs during charging. after they've been charged they behave normally unless the inital charge is exceeded.

Thanks for the advice. I haven't used these group forums ever, so I was unsure what to expect when I checked back. I can't believe the amount of responses I see. Now I will try to explain a little further.

What my application calls for is the use of a ceramic chip (size and temperature characteristics are attractive), to filter high frequency switching noise. What I do not like about them is that they are almost too fast. Fast response times are passing too much current during low voltage - high current demand times. When an electolytic is used, the current through the cap is attenuated considerably (especially in the reverse direction). It's almost as if the "laziness" of the electrolytic is what I desire. I have done a few experiments with adding a bit of esr on the ceramic, and that does help. However, I really think that if there was a way to polarize a non-polarized capacitor, I could more effectively block that reverse current. I could be way off of course because that happens quite a lot, but it never hurts to think a little unconventionally. Thanks for all of the help.

Jason

Yes, I understand your concerns and I have read many of the answers you have received and it is quite obvious that these people do not understand the real problem in the way that we might. Perhaps what you are missing is the concept of low frequency electrochemistry. As you will know electrolytic capacitors need to be fromed before they can be sold to the general community for use in their low technology equipment. Obviously the process of froming requires energy and, in as much as an electrolytic capacitor can be fromed it can also be unfromed. I have personally saved my company lots of money by unfroming electrolytic capacitors where they have been put in the 'wrong' way. It is very important to understand the energies required for unfroming as well as the electrochemistry involved. If you exceed the unfroming energy that is available from the electrochemistry per normalised time then you will damage the capacitor you are trying to unfrome and not recover its full perfromance. You are quite right to believe that there should be a way to make MLCC capacitors frome and unfrome in the same way but I am sure that you will not find the answer here.

DNA

In message , dated Mon, 7 Aug

Indeed; it's only available at a secret location in Somerset, England.

John Woodgate wrote in news: snipped-for-privacy@jmwa.demon.co.uk:

It had me wondering too. A small river of that name disappears not half a mile away from me. I am in Somerset right now, almost.